1. Technical Field
The present invention relates to a reflective optical system and, more particularly, to a compact all-reflective optical system with a wide field of view and very fast optical speed.
2. Discussion
All-reflective optical systems are often utilized in certain imaging applications where chromatic aberrations, thermal behavior, size, weight or cost restricts the use of conventional refractive lenses. Additionally, certain imaging applications require optical systems with large unobstructed two dimensional fields of view in order that particular operations or functions such as navigation, pilotage or driving can be accomplished based on the images provided by the optical system. Further, certain imaging applications require optical systems with expanded light gathering capability. Such optical systems must therefore operate at fast optical speeds.
One such type of telescope is a wide angle large reflective unobscured system (WALRUS) which forms a high quality image of an extremely wide angle object field on a flat image surface. The WALRUS is all-reflective and thus has no chromatic aberrations. An eccentric portion of the rotationally symmetric field of the telescope is typically used, such that no part of the aperture is obscured. The mirrors are surfaces of revolution described exclusively as flats, spheres or conic sections, all of which are easily tested and verified in manufacturing using well known conventional tests. Each mirror shares a common axis of rotational symmetry facilitating telescope alignment. Occasionally, one mirror is used twice providing the function of a secondary and tertiary mirror while eliminating the need to fabricate and align a separate tertiary mirror.
One such type of WALRUS optical system is illustrated in the article entitled "Easily Fabricated Wide Angle Telescope" by R. Calvin Owen, 430 SPIE, Volume 1354, International Lens Design Conference (1990). While this WALRUS provides a wide field of view, it is large and functions at a relatively slow optical speed. The system provides a planar fold mirror and the mirror surfaces are only spherical and conic. Further, the system provides a circular aperture.
Another wide angle large reflective unobscured system is illustrated in U.S. Pat. No. 4,598,981. This patent illustrates a three-mirror system which provides a long telescope. Also, the mirrors of the system are spherical or conic surfaces and the system optical speed is relatively slow.
Thus, there exists a need in the field to provide an optical system which provides a requisite wide angle field of view, is relatively compact reducing the length of the design, corrects all aberrations and significantly increases the optical speed of the system.